If the right inequality holds, we say that \((h_n)_{n\in \mathbb {N}}\)  is a Bessel system. The least constant \(C\)  in (REF ) is denoted by the Riesz constant of the sequence \((h_n)_{n\in \mathbb {N}}\) . Shapiro and Shields proved in [1]} that a sequence \(Z\)  in \(\mathbb {D}\)  is interpolating if and only if the sequence \((\hat{s}_{z_n})_{n\in \mathbb {N}}\)  of normalized Szegö kernels at the points of the sequence is a Riesz system. Moreover, one can find in [2]} a proof that, given any reproducing kernel Hilbert space \(\mathcal {H}_k\)  on a domain \(X\)  and given a sequence \((x_n)_{n\in \mathbb {N}}\)  of points in \(X\) , then the measure
\(\mu _X:=\sum _{n\in \mathbb {N}}||k_{x_n}||^{-2}_{\mathcal {H}_k}\delta _{x_n}\) 
